Surface modifications of rolled W during exposure to high heat loads with He

Background: Tungsten (W) is a promising candidate for future use in fusion reactors as plasma facing material. During operation it will be exposed to both particle irradiation and high heat flux (HHF) loads. Particle irradiation, especially helium (He), is expected to cause significant surface modifications of W, might leading to performance degradation. The effect of heat loading can accelerate such surface damage. Therefore, the performance of W under the combined action of both heat and particle fluxes has to be investigated in more detail. Purpose: The aim is to investigate the surface modifications of rolled W during exposure to combined heating and He particle flux depending on surface temperature, He flux and fluence. Methods: The He-flux/heating experiments were performed on rolled W samples using the neutral beam HHF test facility GLADIS at IPP Garching, Germany. HHF loads generated with He atoms of 2.4 MW·m^-2 and 9.5 MW·m^-2 were used. W samples were adiabatically loaded to peak surface temperatures of ∼1000 K, ∼2000 K and ∼3000 K, close to melting. He fluences increased from about 1 × 10²¹m^-2 to 6 × 10²² m^-2. Results: During He-loading at a low surface temperature (∼1000 K), with increasing fluence, defects appeared in grains first, and then blistering was observed on the sample surface, with many ruptured blisters to form lids and grooves. The largest blisters formed on grains with near surface normal. As fluence was increased continuously, small holes were formed on the surface, and the grooves still existed. However, at intermediate surface temperature (∼2000 K), with increasing fluence, the damage morphology changed from a porous structure with blisters, via a porous structure with grooves, to a small coral-like structure. At high surface temperature (∼3000 K), an evolution from a porous structure to a coral-like structure with increasing fluence was observed, and the coral-like structure changed little as the increasing of fluence continuously. It is shown that the surface modification of rolled W is dependent on a competitive relationship between the formation of blisters and coral-like structures. To study the behaviour of such He damaged surfaces under fusion relevant transient heat loads, 0.9 GW·m^-2 Edge Localized Modes (ELMs)-like laser thermal shocks were applied. Local melting was observed on the coral-like structure after thermal shocks. Conclusion: The damage morphology of rolled W changes from blisters with a grain orientation dependence to a porous/coral-like structure with increasing temperature, because at high temperatures the He_n and He_nV_m (He-vacancy) clusters will be broken up and He will diffuse relatively deeply into the W bulk. This transformation of damage morphology can also be induced by the rise of He fluence. (authors)